For many years, electrical insulators intended for usage on high voltage transmission lines were made of porcelain. In recent years, it has been discovered that such insulators may be formed from certain polymer-based concrete compositions to provide significant advantages in cost and performance. U.S. Pat. No. 4,210,774, issued Jul. 1, 1980, for example, discloses such a polymer filled concrete composition used to produce insulators having dielectric and mechanical characteristics far superior to those of porcelain insulators. The '774 patent discloses that polymer concrete insulators may be formed by generally conventional casting or molding techniques, wherein the mold is simultaneously vibrated and subjected to a vacuum during the curing process to extract entrained air from the composition. It is well known that increased porosity through air entrainment adversely affects the dielectric and mechanical strength of the insulator, encouraging the formation of electrically charged areas within the body of the insulator which may eventually lead to failure.
The use of a vacuum to degas products during the molding process is well known in the art. U.S. Pat. Nos. 3,154,618, issued Oct. 27, 1964, and 4,256,444, issued Mar. 17, 1981, illustrate typical examples of casting and injection molding methods and apparatus known in the art, wherein the mold itself is maintained under a vacuum during at least part of the curing process. Each time the mold is removed from conventional apparatus, the vacuum must be broken and re-established for the next molding cycle. No known method or apparatus allows multiple molding cycles to be carried out without repeatedly subjecting the mold to alternating cycles of vacuum and atmospheric pressure. It has been recognized that substantial amounts of time and energy are required in order to repeatedly subject a mold to a vacuum suitable for degassing each production cycle.
Polymer concrete typically comprises the combination of one or more aggregate materials, a polymer resin, and a catalyst which, when combined with the polymer resin, forms a catalyzed composition hardenable at room temperature. It is known in the art that casting methods and apparatus for use with such compositions must be specially adapted to avoid undesirable clogging caused by premature hardening of the composition. U.S. Pat. No. 2,862,239, issued Dec. 2, 1958, for example, discloses injection molding processes and apparatus wherein catalyst and resin are combined immediately prior to entering an injection pipe leading to a mold, with solvent tanks operatively connected to clean the system of the rapidly setting composition after each injection. The mixing means disclosed in the '239 patent, however, is suitable only for combining relatively low viscosity constituents, and is inadequate for thoroughly combining the high viscosity resin/aggregate mixture with a low viscosity catalyst used to produce polymer concrete.
It is also well known in the art of casting and molding to employ a mold having a generally rigid outer structure, with a generally pliable liner forming the interior cavity configured to receive the molding composition. Room temperature vulcanizing silicone rubber (RTV) is commonly used for creating molds, as disclosed in U.S. Pat. Nos. 4,098,856, issued Jul. 4, 1978, and 3,989,790, issued Nov. 2, 1976. Conventional RTV-lined molds used for casting concrete polymer insulators typically comprise two substantially identical halves, having essentially planar mating surfaces which allow an unacceptable amount of flash to form on the finished product. The unusually high degree of flash formed on concrete polymer insulators requires a significant amount of hand finishing to remove, a time consuming process which frequently increases the cost of the end product. Numerous solutions to this problem have been attempted, but none found to be commercially feasible.